Probably shortly after Superman first moved to the North Pole. The Claus’ were thrilled to have a neighbor after centuries of living alone.

Mrs Claus baked some cookies and brought them over to Kal shortly after he completed his fortress. In return, he exposed a bunch of Santa’s reindeer to Kryptonian red sun rays, which enabled the reindeer to fly. However, one of them wandered onto the grounds of the fortress and unwittingly got exposed to Red K. This turned his nose red but there were no ill effects.

You won’t find THAT in continuity anywhere, but it certainly should be.